1. Field of the Invention
The present invention relates to a thermal displacement compensating device for a machine tool, and more particularly, to a device configured to adjust a thermal displacement compensation amount based on a compensation error in response to a thermal displacement change state, thereby precisely compensating thermal displacement.
2. Description of the Related Art
In a machine tool, a feed screw and a spindle are driven by a motor, so that they are expanded to displace the machine position by heat from the motor, frictional heat produced by the rotation of a bearing, and frictional heat from an engagement portion of the feed screw between a ball screw and a ball nut. Thus, the relative positions of a workpiece to be positioned and a tool are shifted. The displacement of the machine position due to the heat causes a problem in performing high-precision machining.
The displacement of the machine position due to heat is conventionally removed by a technique in which thermal displacement is easily compensated at low cost without using a temperature sensor, a technique in which a command position is compensated based on displacement or temperature detected by a displacement or temperature sensor, or a structure that applies initial tension to a feed screw and is not affected by thermal expansion.
A technique disclosed in Japanese Patent Application Laid-Open No. 2002-18677 is characterized in that thermal displacement can be easily compensated at low cost without using a sensor. The position of a feed shaft is detected, and an average movement speed is determined by the detected position. Based on this, an amount of thermal displacement due to heat generation, heat dissipation, and heat conduction from adjacent sections of the entire stroke of the feed shaft is estimated for each section, and the thermal displacement amounts for the individual sections from a reference point to a compensation position are added up. In this way, the distribution of thermal displacement in each position of a feed screw can be estimated, so that the thermal displacement can be precisely compensated in any position. Further, the thermal displacement can also be precisely compensated in consideration of thermal displacement due to heat generated by the rotation of a spindle or spindle motor, as well as of the feed shaft. Furthermore, more accurate compensation can be performed by compensating a heat-generation coefficient in a thermal displacement amount calculation formula, based on an amount of deviation (compensation error) between an estimated thermal displacement amount (compensation amount) and an actual machine position.
Further, Japanese Patent Application Laid-Open No. 9-85581 discloses a technique in which a temperature sensor is used and a method of estimating the thermal displacement of a spindle is varied depending on states, that is, a steady state at a saturated temperature and a transient state after a change of the spindle speed, based on a temperature detected by the temperature sensor.
If the machine tool is actuated, the spindle and the feed shaft are drastically elongated by an increase in temperature due to heat generation. As heat generation and dissipation are balanced thereafter, the rate of elongation is decreased and becomes substantially constant. If the drive is stopped or slowed down, in contrast, the shafts contract due to heat dissipation. Thus, the change of the thermal displacement amount of the machine tool can be classified into the following three states (see FIG. 1):
(1) a heat generating state in which the thermal displacement amount increases due to heat generation,
(2) a steady state in which heat generation and dissipation are balanced so that the thermal displacement amount is substantially fixed, and
(3) a heat dissipating state in which the thermal displacement amount decreases due to heat dissipation.
Japanese Patent Application Laid-Open No. 2002-18677 described above discloses a method of correcting the heat-generation coefficient in the thermal displacement amount calculation formula based on the correction error. Since the calculation formula includes additional coefficients (a heat-dissipation coefficient and a heat conduction coefficient for calculating heat conduction from adjacent sections), the precision of the correction may not be able to be satisfactorily improved by only modifying the heat-generation coefficient, in some cases. While the coefficient is modified without regard to the state (heat generating state, steady state, or heat dissipating state), moreover, the correction error is not always substantially constant irrespective of the state. As shown in FIG. 2, for example, an actual thermal displacement amount is substantially equal to a calculated thermal displacement amount in the steady state. In the heat generating or dissipating state, however, there may be some difference between the actual and calculated thermal displacement amounts. According to Japanese Patent Application Laid-Open No. 2002-18677, in such a case, however, proper correction cannot always be achieved in a state without the occurrence of a compensation error.
Further, the technique disclosed in Japanese Patent Application Laid-Open No. 9-85581 is intended to change the method of estimating thermal displacement depending on the state, whether steady or transient, thereby accurately compensating the thermal displacement despite the change of operating conditions. However, the transient state involves a case (heat generating state) where the thermal displacement increases and a case (heat dissipating state) where the thermal displacement decreases, and the thermal displacement does not always change to the same degree in these two states. A compensation error occurs if the thermal displacement changes differently. A method of overcoming the occurrence of the compensation error is not disclosed in the above patent document. Further, the use of the sensor causes a problem of an increase in cost and requires protection against a coolant, chips, etc., to ensure reliability.